Color display device

ABSTRACT

A color display device (10) incorporates three light sources (12) supplying red, green and blue light respectively. A spatial light modulator (20) is positioned to provide time varying selective blocking of light from the sources (12) under television signal control. Light passes from the sources (12) to a condenser lens (16), then through a liquid crystal display element (22) and a projection lens (18) before reaching a screen (30). The display element (22) is also under television signal control. Signals supplied to the display element (22) provide for successive sets of three frames to be displayed, each set incorporating frame information to be displayed as red, green and blue and combinable to form a colored frame. When the element (22) displays a frame corresponding to one of the three colors red, green and blue, the modulator (20) transmits that color and blocks the others. The screen (30) receives a color image in which red, green and blue are displayed successively for each frame.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a colour display device.

2. Discussion of Prior Art

Colour display devices such as colour television and colour projectionapparatus are well known. One variety of colour television involves athermionic vacuum tube containing a phosphor pixel screen. The screenhas three phosphor dots in respective primary colours at each pixel.Three electron guns are employed, each addressing a respective primarycolour dot at each pixel. This arrangement is complex, bulky andexpensive. Colour projection apparatus typically employs a collaredslide projected on to a screen with illumination by a white lightsource. This does not land itself to the projection of televisionsignals, since such slides are only suitable for still images.

Liquid crystal displays are known for the production of televisionpictures, but there is difficulty in producing sufficiently largedisplays of the required quality. To obtain a large area picture, it isknown to employ three liquid crystal displays each illuminated by arespective primary colour. Light from each display is projected on to ascreen at which the primary colours are superimposed to yield a colourimage. This is optically complex, as it requires three optical sectionswith accurate mutual alignment.

Colour display systems incorporating liquid crystal light valves orspatial light modulators are also known. International PatentApplication No. PCT/US88/00142 published as No. WO88/06391 relates toone such system. It includes a white light source and an array ofdichroic mirrors to define separate red, green and blue light beams. Thebeams pass through respective polarizers referred to as opticalswitches; the latter are arranged to provide either polarizationrotation or no such rotation according to an electrical signal inputeffecting an "off" or "on" state. The beams are subsequently recombinedto form a single beam and pass to a polarization analyser. The analyserreflects light with rotated polarization to a light valve and transmitslight having unchanged polarization. The red, green and blue beamsundergo rotation of polarization successively, so that at any one timethe light valve receives light of one of the three colours. A cathoderay tube illuminates the light valve to form an image therein, and theimage reflects the incident red, green or blue light to a projectionscreen or the like. This system is undesirably complex. It requires fivepolarization selective devices and three colour selective (dichroic)mirrors.

European Patent Application No. 0301 715 A1 relates to a display systemin which a display matrix is illuminated by beams of different collaredlight from a light source. The light source may comprise a singlefiltered white light source, or a plurality of collared sources. Liquidcrystal shutters are arranged adjacent the light source and areswitchable between a light blocking mode and a light transmitting modeto allow a single collared light beam to illuminate the display matrix.As the liquid crystal shutters are adjacent to the light source they maybe affected by heating up the source. Additionally, the embodimentemploying the white light source has an efficiency of only 1/9 or 11%.This is because only one-third of the light incident on the shutters isutilised and only one-third of the light utilised is allowed through theshutters.

The published International Application Number WO 90/05429 also relatesto a display system in which a display is illuminated by a white lightsource via liquid crystal shutters with red, green and blue filters toprovide a single collared illumination beam. This device also has adisplay efficiency of only 11%.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an alternative form ofdisplay device.

The present invention provides a colour display device comprising

(a) a display element arranged to provide successive sets of frames ofinformation for optical read-out, the frame sets each corresponding to acommon set of wavelength intervals and being combinable to form a colourimage;

(b) light generating means arranged to illuminate the display elementwith light of the said wavelength intervals to provide optical read-out,the generating means providing for the wavelength intervals to be orbecome spatially dispersed;

(c) spatial light modulating means located at a position where thereappear real images of a light source or light sources incorporated inthe light generating means,

wherein the spatial light modulating means is arranged to receivespatially dispersed light and to provide time dependent selection ofwavelength intervals appropriate to concurrently displayed frames.

The invention provides the advantage that it is a colour display deviceof simple construction requiring only one display element for allwavelength intervals or primary colours. Because the spatial lightmodulating means receives spatially dispersed light, it is possible toaccept the required wavelength interval and reject others withoutrequiring polarizing devices or dichroic elements as in the prior art.

The modulating means is located at a position where there appear realimages of a light source or light sources incorporated in the lightgenerating means. Consequently the modulating means is not affected byany heating of the light source.

The light generating means may incorporate a white light source anddispersing means arranged to disperse light from that source and locatedin a light path between that source and the real images thereof.

The invention may include a projection lens arranged to project an imageof the display element upon receiving means, and the light generatingmeans may include a condenser lens system arranged to illuminate thedisplay element and to provide convergent light from at least one sourceand directed generally towards the projection lens. The condenser lenssystem may comprise two condenser lenses. The light source may be awhite light source, the generating means may include dispersing meansarranged to disperse light from the source to provide the saidwavelength intervals, and the modulating means may be located at aposition where real images of the source are produced by the condenserlens system in respective wavelength intervals.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention might be more fully understood, embodimentsthereof will now be described, by way of example only, with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic plan view of a colour display device of theinvention; and

FIG. 2 is a schematic plan view of an alternative form of colour displaydevice of the invention.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a colour display device of theinvention indicated generally by 10. The device 10 incorporates threelight sources 12R, 12G and 12B (collectively referred to as the "sources12"), which provide red, green and blue light respectively. The greensource 12G is located on an axis 14, which is the optical axis of acondenser lens 16 and a projection lens 18. The red and blue sources 12Rand 12B are equidistant from and on opposite sides of the green source12G, and the sources 12 are arranged in a plane extending perpendicularto the axis 14.

The device 10 also incorporates an electrically controlled, liquidcrystal spatial light modulator 20, located at position 20Q. Thelocation 20Q is in an image plane of the sources 12 formed by the lens16. This image plane is in the vicinity of (but need not be contiguouswith) the projection lens 18.

A transmissive element 22 to be imaged by the device 10 is locatedadjacent the condenser lens 16. The element 22 is a liquid crystaldisplay bearing pictorial information in a television format, thedisplay being suitable for optical read-out by an incident light beam.

The device 10 operates as follows. A television signal is supplied by asource 24 to the transmissive element or display 22. The signal isconstructed of successive signal frame sets, each set incorporatingthree successive frames of information. Each frame set incorporates theimage information required to produce one collared image. Each of thethree frames of information in a set corresponds to a respective primarycolour, red, green or blue. In synchronism with the television signal, adrive signal is supplied by the TV signal source 24 to the modulator 20.The modulator 20 has three sections (not shown) each of which may beoptically opaque or transparent as controlled by the drive signal. Thedrive signal controls the optical states of the modulator sections sothat the red, green and blue sources 12R, 12G and 12B supply light tothe display 22 one at a time and in succession; is when one modulatorsection is transmissive the other two are opaque, and the drive signalswitches the sections from opaque to transmissive in succession. Themodulator 20 therefore acts as a fast, positionally movable shutter.

Red, green and blue light passes to and beyond the display 22 asindicated by chain, solid and dotted lines 28R, 28G and 28Brespectively. The condenser lens 16 forms real images of the sources 12at the location 20Q. The projection lens 18 forms a real image of thedisplay 22 upon a screen 30. Any light ray leaving the display 22 andpassing through the projection lens 18 contributes to the image of thedisplay formed on the screen 30. In consequence, the directions fromwhich the display 32 is illuminated are not critical, and consequentlynor is the collective size of the sources 12, provided it is not toolarge. For the same reason, the distance of the projection lens 18 fromthe condenser lens 16 need not be particularly accurate; this accuracyonly affects light collection efficiency. However, it is important thata modulator 20 is located in an image plane of the sources 12 producedby the lens 16. It is also important that the relative positioning ofthe display 22, projection lens 18 and screen 30 satisfy the well-knownlens equation: ##EQU1## where u=object distance between the projectionlens 18 and display 22;

v=image distance between the projection lens 18 and the screen 30; and

f=the projection lens focal length.

The projection lens 18 is positionally adjustable to ensure that awell-focused image of the display 22 appears on the screen 30.

The signals output from the TV signal source 24 are synchronised so thateach frame of information corresponding to red light in a collared imagereaches the display 22 at the same time as the modulator 20 transmitsred light 28R but blocks green and blue light 28G and 28B. Similarly,green light 28G or blue light 28B is transmitted by the modulator 20(with blockage of the other two colours in each case) at times when thedisplay 22 bears forms of information for illumination in green light orblue light respectively. The TV signal therefore consists of sets offrames in which each set contains a red frame, a blue frame and a greenframe in sequence. Each frame becomes displayed on the screen 30 insuccession in light of the appropriate colour in each case.

The display 22 receives light of all three colours simultaneously, butthe modulator 20 blocks light from two of the three real collared imagesof the sources 12 at 20Q. In consequence, only one of the three coloursat a time reaches the screen 30.

Referring now to FIG. 2, there is shown an alternative embodiment of theinvention in the form of a device indicated generally by 40. The device40 incorporates a white light source 42, which is located in the focalplane of a first condenser lens 44. Light from the lens 44 passesthrough a high dispersion prism 46 to a second condenser lens 48, andthence to a transmissive liquid crystal display 50. Light transmitted bythe display 50 passes through a spatial light modulator 52 and aprojection lens 54, the modulator 52 being located in the focal plane ofthe second condenser lens 48. A screen 56 receives light from theprojection lens 54. It is positioned relative to the lens 54 and display50 in accordance with Equation (1). A television signal source (notshown) supplies successive sets of frame signals to the display 50, andalso supplies a drive signal to the modulator 52.

The device 40 operates as follows. White light from the source 42,indicated by divergent rays such as 58, is rendered substantiallyparallel by the first condenser lens 44. The light 58 is dispersed bythe prism 46, red and blue rays being indicated by solid and chain linesrespectively. Dispersed collared images of the source 42 are produced inthe focal plans of the second condenser lens 48, red and blue imagesbeing indicated by crossings 59 and 60 of solid and chain linesrespectively. These images are superimposed on the modulator 52, whichhas sections (not shown) disposed to block or transmit red, green andblue light images respectively. The dispersion introduced by the prism46 is sufficiently large to ensure that the red, green and blue imagesof the source 42 do not overlap one another to a significant extent.

For a source 42 in the form of a filament lamp with an elongatefilament, the dispersed dimension of the filament image is the filamentwidth or transverse dimension. In consequence, the red, green and blueimages of the filament have substantially parallel longitudinaldimensions which are transversely displaced from one another. Thisprovides two benefits. It allows the use of projection lens 54 which issmaller than would otherwise be the case, and it reduces colour imageoverlap.

The optical transmissions of the display 50 and modulator 52 arecontrolled by a television signal source (not shown) as described inrelation to FIG. 1. In consequence, successive single colour framesappear on the screen as the modulator 52 transmits red, green and bluewavelength intervals in sequence, and the display 50 bears successivesingle colour frames. The device 40 is therefore equivalent to thedevice 10 of FIG. 1 with the substitution of a white light source 42 anda dispersive element 46 for discrete collared light sources 12.

The device 40 may be modified by replacing the prism 46 by a diffractiongrating blazed in an order appropriate to produce adequate dispersion atthe position of the modulator 52. The flat screen 56 may be replaced bya curved screen in a wide-angle device.

The devices 10 and 40 need not be employed with the three colours red,green and blue. It is necessary to employ a plurality of wavelengthintervals giving a reasonable degree of coverage of the visible spectrum(400-700 nm) in order to produce adequate colour images. Whicheverintervals are selected, each television frame must bear image detailsappropriate to the associated wavelength interval illuminating it. Thedevices 10 and 40 may be adjustable for colour control by providing forthe dwell time at different wavelength intervals to be variable.

There is also an alternative approach to colour control; this involves aspatially variable interval produced by the modulator 52, instead of thetemporal equivalent previously described. In this alternative, themodulator 52 is controllable to vary the size of one or more transparentilluminated regions through which light reaches the screen 56. This hasa further potential benefit when used in conjunction with a typicalcommercial light source 42 of finite size. Such a source gives rise to adegree of overlap of differently collared images at the modulator 52.The overlap regions may be reduced or eliminated by the modulator 52with spatial interval variation as described above.

The light usage efficiency of the devices 10 and 40 may be improved byemploying a concave mirror or mirrors to reflect into the firstcondenser lens 44 light leaving the light source 42 to the right in FIG.2. The displays 22 and 50 may be reflection mode devices instead ofbeing transmissive, in which case the devices of the invention wouldhave folded optical paths.

I claim:
 1. A color display device comprising:(a) light generating meansfor illuminating a display element with light of a common set ofwavelength intervals, said light generating means including at least onelight source and means for forming a real image for each wavelengthinterval of said common set of wavelength intervals, at least one ofsaid real images spatially dispersed with respect to at least another ofsaid real images; (b) wherein said display element provides successivesets of frames of information for optical read-out, the frame sets eachcorresponding to said common set of wavelength intervals and beingcombinable to form a color image; and (c) spatial light modulatingmeans, located at a position where said light generating means formssaid real images of said at least one light source, said spatial lightmodulation means is responsive to said spatially dispersed light, saiddisplay element and said common set of wavelength intervals, saidspatial light modulating means for providing a time dependent selectionof said wavelength intervals appropriate to said successive sets offrames.
 2. A device according to claim 1 wherein the light generatingmeans incorporates a white light source and dispersing means dispersinglight from that source and located in a light path between that sourceand the real images thereof.
 3. A device according to claim 1 includinga projection lens for projecting an image of the display element uponreceiving means, and wherein the light generating means includes acondenser lens system for illuminating the display element and forproviding convergent light from said at least one light source anddirected generally towards the projection lens.
 4. A device according toclaim 3 wherein the condenser lens system comprises two condenserlenses.
 5. A device according to claim 4 wherein said at least one lightsource is a white light source, the light generating means includesdispersing means for dispersing light from said white light source toprovide said wavelength intervals, and the spatial light modulatingmeans is located at a position where real images of said white lightsource are produced by the condenser lens system in said respectivewavelength intervals.
 6. A color display device according to claim 1,wherein said light generating means includes a plurality of lights, atleast two of said lights having different wavelength intervals and saiddisplay element is at least partially illuminated by spatially dispersedlight from said plurality of lights.
 7. A color display device accordingto claim 1, wherein said at least one light source is a source of whitelight and said light generating means includes a means for spatiallydispersing said white light into a set of common wavelengths.
 8. A colordisplay device according to claim 1, wherein said spatial lightmodulating means passes a time dependent selection of said wavelengthintervals appropriate to said successive sets of frames.
 9. A colordisplay device comprising:(a) light generating means for illuminating adisplay element with light of a common set of wavelength intervals, saidlight generating means including at least one light source and means forforming a real image for each wavelength interval of said common set ofwavelength intervals, at least one of said real images spatiallydispersed with respect to at least another of said real images; (b)wherein said display element provides successive sets of frames ofinformation for optical read-out, the frame sets each corresponding tosaid common set of wavelength intervals and being combinable to form acolor image; and (c) spatial light modulating means, located at aposition where said light generating means forms said real images ofsaid at least one light source and synchronized with said displayelement, for passing successive frames of a time dependent selection ofsaid wavelength intervals from said display element.
 10. A color displaydevice comprising:(a) a white light source for illuminating a displayelement with light of a common set of wavelength intervals, said displayelement located at least partially within a path of light from saidwhite light source; (b) a prism and lens combination, located within thepath of light, for forming a real image for each wavelength interval ofsaid common set of wavelength intervals, at least one of said realimages spatially dispersed with respect to at least another of said realimages; (c) a display element for providing successive sets of frames ofinformation for optical read-out, the frame sets each corresponding tosaid common set of wavelength intervals and being combinable to form acolor image; and (d) a spatial light modulator, located at a positionwhere said prism and lens combination forms said real images of saidwhite light source and responsive to light from said display element,for providing a time dependent selection of said wavelength intervalsappropriate to said successive sets of frames.